1. Field of the Invention
The present invention relates to a cladding for suppressing vortex induced vibration of underwater pipes, cables or other elongate members.
2. Description of the Background Art
When water flows past an underwater pipe, cable or other elongate member, vortices may be shed alternately from either side. The effect of such vortices is to apply fluctuating transverse forces to the member. Such forces can cause the member to bend more than is desirable and impose unwanted additional forces on the member's point of suspension. If the shedding frequency of the vortices is close to a natural frequency of the member then resonance effects can result in particularly severe and potentially damaging oscillation. The problem is experienced particularly in connection with marine risers of the type used in sub-sea oil drilling and extraction. It is referred to as “vortex induced vibration” or “VIV”.
It is known to apply to elongate underwater members a cladding whose exterior is shaped to suppress VIV. Reference is directed in this regard to UK patent application No. 9905276.3 (publication No. 2335248) which discloses an underwater cladding made up of a number of separately formed sections assembled to form a tubular structure receiving an underwater member and having sharp edged helical strakes along its length which, by controlling transition from laminar to turbulent in a flow of water on the structure, serve to suppress VIV. The sections are moulded from polyurethane and are semi-tubular, a facing pair of such sections being assembled around the underwater member to surround it.
The cladding has proved itself in practice to be highly effective. However there are commercial pressures to produce a unit which is more economical in manufacture than the existing polyurethane cladding. Additionally the existing cladding has moderately thick walls which add to its mass and also to the area it presents to a flow, so that drag is increased. Reducing the mass and frontal area are desirable.